Tunable Telecom Lasers Promote Flexibility First

With the telecom market showing signs of improvement, interest in tunable lasers is growing. Service providers are looking to the technology not just for sparing and inventory replacement, but also to reduce network operating costs in ultralong-haul, long-haul and metro applications by offering fast, flexible switching and reconfiguration. Drivers influencing this trend include the simultaneous high output power, narrow linewidth, low noise, high side mode suppression ratio and high reliability characteristic of the latest generation of tunable devices.

One type of tunable CW laser is somewhat of a smaller cousin of the external-cavity lasers common in test-and-measurement applications. Jeff Hutchins, senior applications engineering manager for iolon Inc. in San Jose, Calif., reports that the basic configuration of the company’s tunable modules combines a laser diode and grating with a tuning element — in this case, a microelectromechanical system (MEMS) mirror. Tilting the mirror essentially changes the angle of light reaching the grating, thus providing wavelength tunability. A potential strength of lasers in this category is their use of low-cost, reliable components, where yield is not necessarily as much an issue as with monolithically integrated tunable lasers.

Other configurations rely on distributed Bragg reflectors. Recent lasers from Bookham Technology plc of Abingdon, UK, fall into this category. Another example is tunable sampled-grating distributed Bragg reflector technology from Agility Communications in Santa Barbara, Calif., which includes sampling front and back gratings longitudinally integrated with active and phase-tuning sections. The firm’s engineers reported that its distributed Bragg reflector structure provides wide tunability across the C-band and into the L-band by combining refractive index tuning through current injection with the vernier effect.

Also of interest are emerging strategies that are based on thermally tuned distributed feedback technology. According to Bardia Pezeshki, chief technical officer of Santur Corp. in Fremont, Calif., one issue with these devices is their tendency toward lower manufacturing yield, which can carry over into tunable laser designs. To resolve this problem, Santur’s manufacturing process fabricates multiple lasers on a chip, which reportedly improves the odds that a chip will yield a good laser. It also relies on MEMS technology to optimize coupling of light into the fiber.

Although the companies in the tunable telecom market space are relatively few at present, the market is growing. Perhaps the latest contender is Two-Chip Photonics of Darmstadt, Germany, which focuses on electrically pumped tunable vertical-cavity surface-emitting lasers (VCSELs). Hubert Halbritter, one of the principal founders, said that the company is an outgrowth of a recently completed Information Society Technologies project.

Halbritter reported that the company recently extended the tuning range to 40 nm, the highest so far that he knows of for electrically pumped VCSELs. The fixed epitaxial top mirror of a nontunable VCSEL has been replaced by a tunable mirror membrane, which is fabricated on a separate chip. This gives designers the option to optimize each component individually.

Halbritter and colleagues observed that, although optically pumped VCSELs still offer higher output power than electrically pumped lasers, they are more difficult to handle from an application standpoint and have more complex packaging because of the need for a pump laser. The current generation of electrically pumped lasers thus comes at a cost advantage. The scientists believe that, although output power is still lower than with edge emitters, the electrically pumped lasers will offer advantages related to continuous, single-mode and mode-hop-free tuning in applications where power may not be as critical.

With all of the different strategies available with tunable lasers, it is surprising to note the recent success at standardization efforts supported by the Optical Internetworking Forum, as well as tunable laser manufacturers and end users.